Video cameras have been miniaturized by the development of semiconductor device technology to become convenient to carry and be widely used. Heretofore, in video cameras, a CCD sensor has been used as a solid-state imaging device. However, the CCD sensor requires a plurality of kinds of voltages for driving the device, and requires a plurality of power source circuits for generating the voltages from a power source voltage. Accordingly, the above-described features of the CCD sensor have inhibited a video camera from being further miniaturized, and has been a factor which inhibits the reduction in power consumption.
Under the above-described circumstances, a MOS sensor in which an amplifier MOS transistor is used is expected as a solid-state imaging device which replaces a CCD sensor. The MOS sensor can be driven by a single power source. Further, in the MOS sensor, power consumption and voltage can be lowered. A cross-sectional view showing the structure of a unit cell of the conventional solid-state imaging device is shown in FIG. 10.
A photodiode 202 is provided in the upper portion of a silicon substrate 201. Further, in the upper portion of the silicon substrate 201, the drain 203 of a readout transistor is provided to be spaced apart from the photodiode 202. A gate wiring layer 204 is provided between the photodiode 202 and the drain 203 on the silicon substrate 201 with a silicon oxide film 205 interposed therebetween. A silicon oxide film 206 is provided in an element isolation area, whereby an element area is electrically isolated.
An interlayer insulating film 207 is provided over the gate wiring layer 204 and the silicon oxide film 205. A microlens 208 is provided on the interlayer insulating film 207, and focuses light incident on the microlens 208 on the photodiode 202.
Next, with reference to FIG. 11(a) to FIG. 11(c), light incident on the photodiode 202 in a unit cell of the conventional solid-state imaging device shown in FIG. 10 will be described. FIG. 11(a) to FIG. 11(c) are views each showing the paths of incident light in a unit cell of the conventional solid-state imaging device. FIG. 11(a) to FIG. 11(c) each show the case where a camera lens 209 is provided over the conventional solid-state imaging device and where light focused by the camera lens 209 is incident on the microlens 208. It is noted that, for convenience of explanation, a description will be made by regarding as the right side the side on which the gate wiring layer 204 is provided, with respect to the photodiode 202, and regarding as the left side the side opposite to the gate wiring layer 204, with respect to the photodiode 202.
FIG. 11(a) shows the case where a unit cell is located directly under the camera lens 209 in an image area and where the optical axis of the microlens 208 and that of the camera lens 209 coincide. Further, FIG. 11(b) shows the case where a unit cell is located on the right side of the optical axis of the camera lens 209 in the image area and where the camera lens 209 is located at the upper left with respect to the microlens 208. Moreover, FIG. 11(c) shows the case where a unit cell is located on the left side of the optical axis of the camera lens 209 in the image area and where the camera lens 209 is located at the upper right with respect to the microlens 208.
First, with reference to FIG. 11(a), light incident on the photodiode 202 in the unit cell located directly under the camera lens 209 will be described. Light passing through the center of the camera lens 209, which is hereinafter referred to as (principal ray), passes through the center of the microlens 208 to be perpendicularly incident on the photodiode 202. On the other hand, light incident on the microlens 208 from the upper right travels in the interlayer insulating film 207 in a direction which deviates from the center of the microlens 208, and is incident on the element isolation area adjacent to the photodiode 202. Moreover, light incident on the microlens 208 from the upper left travels in the interlayer insulating film 207 in a direction which deviates from the center of the microlens 208, and is incident on the gate wiring layer 204. Part of the light incident on the gate wiring layer 204 passes through the gate wiring layer 204 to be incident on the silicon substrate 201 under the gate wiring layer 204. On the other hand, the rest of the light, which does not pass through the gate wiring layer 204, is reflected at the interface between the gate wiring layer 204 and the interlayer insulating film 207, and is incident on the photodiode 202.
Next, with reference to FIG. 11(b), light incident on the photodiode 202 in the unit cell located on the right side of the optical axis of the camera lens 209 will be described. A principal ray travels in an oblique direction from the camera lens 209 located at the upper left to the microlens 208 located at the lower right, and is incident directly on the photodiode 202. On the other hand, light incident from the right side of the camera lens 209 is also incident directly on the photodiode 202, similarly to the principal ray. Further, light incident from the left side of the camera lens 209 is incident on the gate wiring layer 204. Part of the light incident on the gate wiring layer 204 is reflected at the interface between the gate wiring layer 204 and the interlayer insulating film 207, and is incident on the photodiode 202.
Next, with reference to FIG. 11(c), light incident on the photodiode 202 in the unit cell located on the left side of the optical axis of the camera lens 209 will be described. A principal ray travels in an oblique direction from the camera lens 209 located at the upper right to the microlens 208 located at the lower left, and is incident directly on the photodiode 202. On the other hand, light incident from the right side of the camera lens 209 is incident on the element isolation area. Further, light incident from the left side of the camera lens 209 is incident on the gate wiring layer 204. Part of the light incident on the gate wiring layer 204 is reflected at the interface between the gate wiring layer 204 and the interlayer insulating film 207, and is incident on the photodiode 202.
In the conventional solid-state imaging device, as shown in FIG. 11(b), light incident from the right side of the camera lens 209 is incident directly on the photodiode 202 in the unit cell located on the right side of the optical axis of the camera lens 209. On the other hand, in the unit cell located on the left side of the optical axis of the camera lens 209, as shown in FIG. 11(c), light incident from the right side of the camera lens 209 is incident on the element isolation area. Although the light incident on the element isolation area passes through the silicon oxide film 206 to undergo photoelectric conversion in the silicon substrate 201, much of the signal charge recombines in the vicinity of the silicon oxide film 206 and, therefore, is not accumulated in the photodiode 202. Accordingly, in the conventional solid-state imaging device, the unit cell located on the right side of the optical axis of the camera lens 209 has a higher sensitivity to incident light from the right side of the camera lens 209 than the unit cell located on the left side of the optical axis of the camera lens 209.
On the other hand, for light incident from the left side of the camera lens 209, the quantity of light incident on the photodiode 202 is approximately the same whether the unit cell is located on the left or right of the image area, because the light incident from the left side of the camera lens 209 is reflected by the gate wiring layer 204 as shown in FIG. 11(b) and FIG. 11(c).
Consequently, in the conventional solid-state imaging device, a unit cell located on the right side of the optical axis of the camera lens 209 has a higher sensitivity to incident light than a unit cell located on the left side of the optical axis of the camera lens 209.
The sensitivity of a unit cell is reflected as the brightness of a pixel in a captured image. Accordingly, if the sensitivities of unit cells differ depending on the positions of the unit cells in the image area, one edge is brighter but the other edge is darker in a captured image. For this reason, it is being demanded that variations in the sensitivities of the unit cells within the image area with respect to the positions of the unit cells are suppressed.